Cryogenic coolers lift heat from infrared detectors and associated electronic and optical components in applications where space is limited. The cryogenic cooler is often inserted into a Dewar (or housing) onto which one or more detector elements are mounted. Current flight vehicle applications often require that an infrared focal plane array of detector elements be cooled to approximately liquid nitrogen temperatures, i.e., 77 K (−193° C.). Joule-Thomson and Stirling Cycle coolers are the two cooling technologies most often used to provide controlled cooling at such temperatures for size and weight critical applications.
A Joule-Thomson cooler is a device that produces liquid refrigerant by use of a valve (known in the art as a “Joule-Thomson valve”) and counter flow heat exchanger. High-pressure gas is allowed to expand through the valve via an irreversible throttling process in which enthalpy is conserved, resulting in lowering of its temperature. The counter-flow heat exchanger transfers heat from the high-pressure incoming gas to the cooled exiting gas to decrease the enthalpy of the incoming gas and enable liquid production. The simplest form of a conventional Joule-Thomson cooler typically uses a fixed-size orifice near the heat exchanger at the cold end of the cooler such that cooling power is unregulated. The input pressure and internal gas flow dynamics establish the flow parameters of the refrigerant through the cooler.
Joule-Thomson coolers require a supply of highly pressurized gas. In certain applications, such as flight vehicles, this requirement tends to impose significant logistical constraints. In the flight vehicle context, for example, only a limited amount of pressurized gas can be carried on the flight vehicle and the host vehicle. This limits the operating time of the weapons platform in proportion to the amount of pressurized gas onboard. After each mission, the gas containers must be recharged. Extended use of Joule-Thomson coolers increases the possibility of contamination due to small impurities that may block the fine orifice of the device. These constraints impose significant field maintenance requirements that limit the viability of Joule-Thomson cryostats for such demanding applications.
Various improvements to the Joule-Thomson technology have been implemented to increase system operating time. These include various mechanisms to throttle back flow after cool down or systems that include a compressor instead of a gas bottle. Use of a compressor in place of a bottle can eliminate the need to recharge the gas container after each mission but comes with its own limitations and field maintenance requirements.
For these and other reasons, Stirling cycle coolers tend to be preferred for such applications as they can run for thousands of hours on electrical power alone. A Stirling cycle cooler is an efficient and compact closed-cycle, electrically-driven cryogenic cooling device using a repeating reversible expansion of a gas. It consists of two main assemblies, a compressor and a cold head. The compressor provides a sinusoidal pressure wave to the cold head. Modern cold heads typically consists of an expansion piston and integral regenerative heat exchanger. The regenerative heat exchanger (regenerator) is included in the expansion piston to thermally isolate gas at the compressor headspace from gas at the expansion headspace. Refrigeration is provided at one end of the cold finger portion of the cold head.
Stirling coolers are inherently slow to bring their thermal load to operating temperature but can run for long periods with only electrical power. Joule-Thomson coolers are much quicker to bring their load to operating temperature but require large amounts of size and weight to run for long periods. Various improvements to the Joule-Thomson technology have been implemented to increase system operating time but these systems do not approach the operating times of a Stirling system. The physics of a Stirling cooler do not allow it to match the refrigeration power and quick ready time of a Joule-Thomson system in any reasonable size limit. The system designer has had to choose between the two types of coolers and compromise vehicle performance as a result.